The use of radioactive material for both energy generation and military purposes has become internationally prevalent over the past few decades. While being inherently dangerous, the destructive potential of radioactive material only becomes apparent when it is taken out of a controlled environment, as a result of nuclear power plant malfunctions, acts of nature or when radioactive material falls into the hands of unauthorized parties. When any of the above occurs, scenarios of a nuclear reactor breach (i.e. Chernobyl and Fukushima), a radioactive ‘dirty’ bomb in the hands of terrorists or even a nuclear bomb attack, become disturbingly realistic (Department of Homeland Security, National Planning Scenarios, March 2006). These risks are further fueled by the increasing number of nuclear power plants that are being built by the newly developing economies of China and South Korea, and the ongoing race of unstable regimes to obtain nuclear arms. Thus, there is an ever-growing risk of nuclear catastrophe:
“Two decades after the end of the Cold War, we face a cruel irony of history. The risk of a nuclear confrontation between nations has gone down, but the risk of nuclear attack has gone up.”—President Barack Obama Apr. 13, 2010
The fundamental danger of radioactive material is the ionizing radiation that it emits. Ionizing radiation damages living tissue by transferring energy to the atoms and molecules in the cellular structure, causing them to become ionized or excited. This which are responsible for vital cell processes.
Cells are able to repair damage in cases where low doses are received, such as from daily background radiation. At higher levels of radiation, cell death results. At exceedingly high doses, functional cells that are lost as part of normal tissue turnover are not replaced because of damage to the stem-cell compartment, leading to tissue failure. Radiation damage is most prominent in tissues with rapidly proliferating cells, as DNA damage from radiation commonly leads to the death of cells in the first cell division after irradiation, or within the first few divisions [.
Acute Radiation Syndrome (ARS), also known as Radiation Sickness, is a serious illness that manifests itself when the human body receives a high dose of ionizing radiation over a short period of time (usually several hours). Many survivors of the Hiroshima and Nagasaki atomic bomb detonations in 1945, and many of the firefighters who first responded to the Chernobyl nuclear power plant accident in 1986, became ill with ARS according to the Centers for Disease Control and Prevention (CDC).
The probability of survival of those inflicted with ARS decreases with escalating radiation dose. Most of the people who do not recover from ARS will die within a few weeks to a few months after exposure, with the primary cause of death being the destruction of the person's bone marrow.
Bone marrow is comprised of hematopoietic stem cells (HSCs) which are ultimately responsible for the constant renewal of blood, giving rise to billions of new blood cells each day. Due to their high rate of proliferation, HSCs are especially vulnerable to ionizing radiation]. Owing to their central role in blood production, lethal irradiation of HSCs may lead to death from severe anemia, infection and internal bleeding. This cause and effect relationship between high doses of radiation and HSC apoptosis, has led to the use of HSC transplantation (i.e. bone marrow transplantation) as a life-saving intervention in cases of exposure to high doses of radiation.
People inflicted with ARS can benefit from bone marrow transplantation post-exposure in an effort to restore their body's cell production, however, this is a challenging procedure. There are two methods of HSC transplantation. In the first, bone marrow is harvested from the individual prior to irradiation, stored and later transplanted. This form of transplantation does not require tissue matching and is thus coined autologous transplantation. However, very few individuals have preemptively stored their bone marrow, making this method common only in myeloablative cancer therapy. The second method is allogeneic bone marrow transplantation, wherein the source of bone marrow is from a donor. Allogeneic transplantation requires that the donor and recipient be of matched tissue types (i.e. human leukocyte antigen types). If mismatched bone marrow transplantation is attempted, graft rejection is likely to ensue. There is also a high likelihood of Graft vs. Host Disease (GVHD), an often fatal condition resulting from an assault of the donor immune cells embedded in the graft on the recipient's bodily tissues. Unfortunately, matched donors are a scarcity due to the tremendous polymorphism in the human leukocyte antigen locus. Moreover, transplantation must take place in the immediate days following exposure to radiation, further increasing the challenge in locating a matched donor. In the event of a catastrophe with a large number of victims, the time-frame imposed is not likely to enable isolation of matched donors.
Based on previous nuclear disasters, such as the atomic bombings of Japan and the Chernobyl meltdown, the median lethal dose (LD50) of radiation in the human population has been established to be around 400 rad (4 Gray or Gy). For those few individuals who are able to obtain matched or otherwise engraftable bone marrow transplants, the LD50 (dose at which 50% of the population perishes) increases to at least 1,000 rad, a fact that is readily demonstrable in medical practice, where thousands of individuals to date have undergone supra-lethal Total Body Irradiation (TBI) for purposes of cancer therapy, and were subsequently rescued by bone marrow transplantation. Thus, life-threatening damage may be reversed by bone marrow transplantation in individuals receiving radiation doses as high as 1,000 rad. Indeed, according to the CDC, at doses between 200 and 1,000 rad, the only significant life-endangering threat is bone marrow damage. At doses in excess of 1,000 rad, damage to gastrointestinal (GI) tissue may become a limiting factor in survival.
According to official estimates of the US Department of Homeland Security (DHS), the majority of deaths resulting from a possible nuclear detonation in an urban area would be from exposure to high doses of external radiation. According to these estimates, most of these deaths would result from doses inside the 200-1,000 rad range, the range in which bone marrow is the only body tissue likely to sustain irreversible damage (see Appendix)
In the Chernobyl disaster, the vast majority of firefighters who were first on the scene, received radiation doses that ranged between 80 and 1,000 rad, according to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) final report on the disaster. Thus, in marked resemblance to the DHS estimates of exposure in a detonation, most exposures in Chernobyl were within a dose range whereby bone marrow was the only seriously damaged tissue. Indeed, UNSCEAR reports have repeatedly concluded that the underlying cause of death among the 28 firefighters who succumbed to ARS in Chernobyl was bone marrow failure [21, 22].
First-responders (firefighters, medics, technicians, etc.) are relied on to perform their life-saving duties, and to stem the exacerbation of the already dire circumstances. The extremely volatile nature of nuclear reactor incidents, such as Chernobyl or Fukushima, requires that first-responders have at their disposal the optimal protection from radiation, so that an already catastrophic scenario does not escalate further.
Ionizing radiation can be classified into two categories: photons (gamma radiation and x-rays) and particles (alpha and beta particles). Shielding the human body from gamma rays requires large amounts of high-mass material, in stark contrast to alpha particles that can be blocked by paper or skin, and beta particles that can be blocked by foil. Gamma rays are best blocked using materials possessing high atomic numbers and high density. For this reason, a lead shield is significantly better (by 20-30%) as a gamma ray blocker, when compared to an equal mass of an alternative shielding material such as aluminum, steel, concrete, water or soil. The higher the energy of the gamma rays, the thicker the shielding that is required.
In nuclear disasters, radioactive materials are commonly presented in the form of nuclear fallout. Fallout consists of radiation-emitting particles ranging in sizes, together creating a cloud of radioactive dust. The closer to ground an atomic bomb is detonated, the more dust and debris is thrown into the air, resulting in greater amounts of nuclear fallout. Whenever individuals remain in an area contaminated with fallout, such contamination leads to immediate external radiation exposure as well as possible later internal hazard from inhalation and ingestion of radiocontaminants. The radioactive dust emanating from nuclear accidents or from radiological bomb (dirty bomb) detonations poses dangers similar to that of fallout.
Protective clothing can protect from the alpha and beta radiation emitted from fallout, but offers no protection from gamma radiation. Facemasks, eye-goggles and respirators can protect from inhalation and ingestion of fallout particles, but again provide no protection from gamma radiation. Gamma radiation attenuating devices and garments have been described in the past, but most were developed to offer either whole-body protection, or to cover as much of the body as possible. Moreover, due to the significant weight of radiation-attenuating materials, existing shielding solutions are made using only a thin layer of radiation-attenuating material, so as to remain bearable by their wearers. These thin layers attenuate radiation transmission in a manner that may be beneficial for reducing the incidence of long-term health effects from low-energy radiation, but are insufficient for preventing the acute health effects that result from exposure to high doses of radiation (i.e. Acute Radiation Syndrome). The resultant loss of active bone marrow in individuals with ARS has dire health consequences. Thus, there is an urgent need for measures that may be taken to protect bone marrow from gamma radiation.